Electronic radiation sensors have been used in the past for many purposes. They have been commonly used for image sensing in television and video tape cameras, in robotic vision, in aerial reconnaissance, and in document scanning, such as that performed in facsimile machines.
An important prior art electronic image sensing devices is the vidicon television camera tube. The vidicon has a transparent conductive plate at one end. Coated on the transparent conductive plate is a photoconductive layer, upon which a light image is focused through the plate. At the other end of the tube is an electron gun which shoots a beam of electrons in a raster scan upon the opposite side of the photoconductive layer from that touching the transparent plate. Where the image focused on the photoconductive layer is bright, the layer is made relatively conductive, and thus, when the electron beam hits that portion of the layer, its current passes through to the transparent plate on the opposite side, generating a signal indicating the presence of light. On the other hand, where the image is dark, the photoconductive layer remains relatively insulative, preventing the electrons projected upon it from passing through to the transparent conductive plate. As a result, the signal produced at the transparent conductive plate varies as a function of the light intensity of the portion of the image being scanned by the electron beam.
The plumbicon is another type of television camera tube which is similar to the vidicon. But instead of having a simple photoconductive layer, the plumbicon has a PIN photodiode layer formed of three sublayers: a thin P layer in which the majority charge carriers are holes, a thin N layer in which the majority charge carriers are electrons, and a thick central intrinsic layer separating the P and N layers in which there are few charge carriers, but in which the number of holes and electrons is relatively equal. The interfaces between the P and I and the I and N layers form rectifying junctions which tend to prevent negative charge on the P layer, upon which the electron beam is projected, from flowing to the N layer, which either contacts or forms the plumbicon's transparent conductive plate. The electron beam places a capacitive charge across each portion of the PIN layer which it scans. However, when light is focused on a portion of the PIN layer, electron-hole pairs are generated in the intrinsic sublayer, which tend to neutralize the charge placed across that portion of the photodiode layer. As the electron beam makes successive scans, it recharges portions of the P layer discharged since the previous scan. This recharging of the P layer generates a current flow in the transparent conductive plate on the other side of the photodiode by means of capacitive coupling. It is this current flow which forms the video signal. The charge upon a given portion of the photodiode layer tends to be discharged by all the light which hits that given portion between electron scans. Thus the amount of current produced when that given portion is read is a function of the time integral of the amount of light which has fallen on that portion since its last scanning, providing increased sensitivity to light.
Although vidicons and plumbicons produce excellent images, they tend to be relatively bulky, fragile, and expensive because they are vacuum tubes. Thus there has been a demand for a solid-state sensing device. One such device provided by the prior art is the charged-coupled device, or CCD. Charge-coupled devices usually comprise a layer of relatively conductive semiconductor material, such as a layer of material which has been doped to be N type. This layer is usually separated from a layer containing electrodes by an insulator. In a two dimensional charge-coupled image sensing array, an x-y array of transparent imaging electrodes receive a voltage during a integrating period in which an image is to be produced. When light hits the semiconductor, it generates charge carriers, which are attracted by the voltage on the imaging electrodes to the portion of the conductive semiconductor layer below those electrodes. The amount of charge which accumulates under an imaging electrode during a given integrating period corresponds to a time integral of the amount of light which falls on the semiconductor under that imaging electrode during that integration period. When the time comes to read the information produced under a row of such imaging electrodes, the individual packets of charge collected under the imaging electrodes of the row are shifted in parallel, by applying voltage to adjacent electrodes, into a charge-coupled shift register, which then serially shifts the packets to an output device, producing a video signal.
Such charge-coupled devices have many advantages. They are solid-state and thus relatively small and rugged. They also have the advantage of having the position of their photosensitive elements determined in a fixed manner by the location of their imaging electrodes, rather than being determined in a less reliable dynamic manner by the electronic deflection circuitry used in video tubes of the type described above.
Although the charge-coupled device technology provides a suitable video sensor for many applications, it does not appear suited for all applications. For example, the signal produced in its individual photosensitive elements is not randomly addressable, since the signal contained in a given line of such photosensitive elements is read out serially in a shift register. Additionally, charge-coupled devices have traditionally been made on crystalline substrates. Because of the present limitations on the size in which such crystalline substrates can be manufactured, and because of the large cost of such crystalline substrates, charge-coupled device technology is not suited for large area arrays of image sensors, such as those which might be used in contact type document copiers.